Another primary consideration in introduction & eventual disappearance of Segmentata?

[Urselius]

The Romans were, throughout their history, economically naive. They never got a handle on monetary inflation, those periods when inflation was not an issue were the result of luck rather than management.

I think that the opposite was true, that late imperial generals, were more skilled, on average, than those of the Principate. This is simply a reflection of the greater elaboration of later armies. The Pricipate army had a uniform basis of reliable heavy infantry, backed by some light and specialist troops and a rather weedy, and also more-or-less uniform, cavalry. It was a relatively flexible but simple army to direct on the battlefield. The late army had more types of specialised troops: heavy infantry had a greater variety of weapons available, therefore  a greater variety of deployment options, and archery was of greater prominence than it had been earlier. Cavalry had become more diverse, with the introduction of super-heavy shock cavalry and horse archers. The late army had a greater similarity to the army of Philip and Alexander of Macedon in its complexity and reliance on combinations of troops with specialised battlefield roles, than it had to the army of the Principate. A more complex army requires a higher level of generalship.